Physics + Math

19
Sep

Beams of Light Could Steer Future Spaceships

Picture this: A spaceship—nothing out of the ordinary, except for the spinning glass rods mounted at the corners—that can be steered by nothing more than light.

While that application may be a long ways off, physicists at MIT and Northeastern University theorize that the Bernoulli principle—the same thing that lifts airplanes and hooks golf balls—may apply not just to objects in fluids like air, but objects sitting in a fluid-like flow, such as a beam of light.

new-horizons
The New Horizons spacecraft is directed by thrusters, but future spacecraft could be steered with light.

The Bernoulli principle says that increased speed of a fluid decreases its pressure. On aircraft, air moving over the wing moves more quickly, creating low pressure, while air moving under the wing moves more slowly and thus has higher pressure, lifting the plane. In golf, a square hit creates backspin, which raises the pressure on the underside of the ball, holding it aloft. Sidespin causes the ball to turn. Tennis balls also rise and veer in response to the same forces, as do soccer balls.

Physics arXiv Blog:

Movassagh and Johnson ask whether a similar difference in pressure might arise for a spinning object in a beam of light. They conclude that it does but only if the object is made of a dielectric material, like glass or plastic.

In a dielectric, am external electromagnetic field can penetrate a short depth into the material. When the material is rotating, this interaction generates a force. Movassagh and Johnson calculate that this force is in the same direction as Bernoulli’s force when the material’s electric susceptibility is positive and in the opposite direction when the electric susceptibility is negative.

Photons act almost exactly like gas molecules in the air, provided the material can interact with them. That’s why dielectrics like glass work; conductors, such as metals, won’t work because they can’t disrupt the electromagnetic field to create the difference in pressure.

The resulting force is extremely small, but it could be magnified using Mie resonance, a phenomenon related to Rayleigh scattering, a principle you’re also familiar with if you’ve ever seen a fiery sunsets. Right now, a light-based Bernoulli principle is all theoretical, but it means it’s possible that our next Voyager-like spacecraft could be steered using rays of starlight.

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